1. Field of the Invention
The present invention relates to an elastic shaft joint (hereafter referred to as a joint) that connects two coaxial shafts in a state having an elastic play in a rotation direction.
2. Description of the Related Art
Such a kind of joint is used, for example, in a steering mechanism of an automobile as shown in FIG. 9. This joint shuts off vibration transmission from a wheel to a steering wheel and allows a steering wheel operation to have an elastic play by utilizing the torsion of a damper.
Referring to FIG. 9, a steering wheel 50, a steering shaft 51, an input shaft 52 of a steering gear device (power steering device), universal joints 53, 54, and a joint 55 are shown.
A conventional joint 55 is disclosed, for example, in Japanese Utility Model Publication No. 58-50105/1983, the contents of which will be hereafter explained with reference to FIGS. 10 and 11, wherein FIG. 10 is a perspective view in an exploded state, and FIG. 11 is a section view.
The joint 55 includes a tube shaft (pipe shaft) 56 being integral with a yoke (not illustrated), a solid shaft 57 inserted in the tube shaft 56, and an elastic body 58 interposed therebetween.
Protrusions 56a to 56c outwardly protruding in the radial direction are disposed at three places on a circumference of the tube shaft 56. Three projections 57a to 57c engaging with the inside of the three protrusions 56a to 56c of the tube shaft 56 with a gap in the rotation direction are disposed at the end of the shaft 57. The elastic body 58 is bonded by vulcanization to both circumferential sides of the first and second projections 57a, 57b and to regions between the three projections 57a to 57c.
The elastic bodies 58 on both sides of the first projection 57a are in contact with inner wall surfaces of the corresponding first protrusion 56a. However, the elastic bodies 58 on both sides of the second projection 57b are disposed to face the corresponding second protrusion 56b with slight gaps C.sub.1, C.sub.1. Further, large gaps C.sub.2, C.sub.2 are disposed between the third projection 57c and the corresponding third protrusion 56c.
Referring to FIG. 12, the characteristics of the torsion spring constant brought about by this structure are as follows. The joint has a low spring constant in a minute steering angle range from A to B due to elastic deformation of the elastic body 58 disposed between the first projection 57a and the protrusion 56a; the gap C.sub.1 of FIG. 11 disappears when the steering angle reaches B; and the joint has a high spring constant in a steering angle range from B to C due to elastic deformation also of the elastic body 58 between the second projection 57b and the protrusion 56b. When the steering angle reaches C, the gap C.sub.2 of FIG. 11 disappears and the torque is directly transmitted. In other words, as a sense of steering operation, when the rotation is started, a gradually increasing elastic reaction is felt in accordance with the increase in the rotation angle, and finally a rigid reaction is felt. This facilitates the steering operation.
The above-mentioned joint has a structure such that the transmitted torque is changed in three steps in accordance with the thickness and presence of the elastic body 58 between the projections 57a to 57c and the protrusions 56a to 56c at three places on the circumference. Therefore, the torque transmitted in each of the gaps between the projections 57a to 57c and the protrusions 56a to 56c is changed, so that an inclination of the tube shaft 56 or the shaft 57 in the rotation direction is liable to be generated. Due to this reason, it is sometimes difficult to obtain a spring constant as designed.
Moreover, since the tube shaft 56 and the shaft 57 have complex shapes, it requires labor in producing the joint 55, and the production costs are high.